Actually some chipsets are using standard 40mm by 10mm fans. The bearing shaft is around twice as long as VGA fans typically allow, making the blade more stable and preserving the life of the fan. Also, the blades are deeper, so the fan doesn't have to spin as fast to deliver the same amount of air. So when you combine the longer shaft with slower speed, you end up with a chipset fan that can last several years, rather than several months.
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